External-rotor reluctance motor

ABSTRACT

A external-rotor reluctance motor has a rotor provided with a plurality of pole portions. Each pole portion has respective pole ends, a pole span between said ends, and a pole middle. The rotor has a given effective yoke cross-section over part of a pole span and a lesser effective yoke cross-section in the vicinity of the respective pole middle and over substantial portions of the associated pole span. The novel reluctance motor exhibits greatly improved pull-in and pull-out torque.

' United States Patent 1 Burgbacher mi "3,775,626 [451, Nov. 27, 1973 EXTERNAL-ROTOR RELUCTANCE MOTOR [75] Inventor: Martin Burgbacher, St. Georgen,

Black Forest, Germany [73] Assignee: .Papst-Motoren KG,

Georgen/Schwarzwald, Germany [22] Filed: Jan. 27, 1972 [21] Appl. No.: 221,275

30 Foreign Application Priority Data Jan. 29, 1971 Germany P21 04 189.4

[52] US. Cl 310/67, 310/162, 310/261, 310/163 [51] Int. Cl. H02k 19/14 [58] Field of'Search 310/67,]162-464, 310/2ll,261

[56] References Cited UNITED STATES PATENTS 3,113,230 I 12/1963 Linkous 310/162 3,002,118 Papst .L 310/67 X 3,596,121 7/1971 Chang..... 310/67 X 2,483,848 10/1949 Saretzky 310/162 3,054,009 9/1962 Papst 310/163 X 2,733,362 10/1956 Bauer et a1. 310/162 Primary Examiner-D. F. Duggan Attorney-Michael S. Striker [5 7] ABSTRACT 1 A external-rotor reluctance motor has a rotor provided with a plurality of pole portions. Each pole portion has respective pole ends, a pole span between said ends, and a pole middle. The rotor has a given efvfective yoke cross-section over part of a pole span and a lesser effective yoke cross-section in the vicinity of v the respective. pole middle and over substantial portions of the associated pole span. The novel reluctance motor exhibits greatly improved pull-in'and pull-out.

torque. 1

19 Claims, 16 Drawing Figures 1;, EXTERNAL-ROTOR-REILUGTANGEEMOTOW BACKGROUND OFTI-IE INVENTION The present. invention relates to synchronous machines, and more specifically to synchronous motors.- In

particular, theinvention relates to reluctancemotors having an external rotor.

Reluctance motors operate synchronously and ar already known. They are used m'oreand more'widely in industrial applications because they provide a highly constant able speed output over a-rangeof load-torqueconditions, and are thus suitable 'formany; applications where constant speed control isof importance; his not thought necessary to explain in detail the concept-and operation of the reluctance motor per se.

The best known reluctance motor construction. makes use of salient'poles, each pole-beingassociated. with an interpolar space. However, with 'suchconstruction' it isnot possible to'guarantee' a predetermined ratio .of direct axis reactance to quadraturefaxis reac= tanceyand the efficiency and usefulness of 'the motor are accordingly somewhat limited.

Accordingly, recent developments haveincluded development of external-rotor reluctance motors in which salientpoles are not employed; andin which=the= peripheral air gap between stator and rotor isaccordingly substantially constant. In addition, various attempts have been made'to-improve therotors of such external-rotor reluctance machines.

One expedient whichhas recently come into use is to provide in the rotor yoke, radially outwards of the conductor-carrying slots, a cavity accommodating a non-magnetic.portion, substantially in the vicinity of a pole middle. Often, such cavities areformed by the alignment of voids of holes stamped into the individual:

the stamped, or

advantages still prevail and call for improvement. In

particular, these relate to the pull-in and pull-out torque characteristics heretofore" achieved. As is known, the pull-in torque is that torque which the motor can produce during start-up, thereby reaching synchronous operation despite" initial loading. Likewise, the pull-out torque is that'torque which causes a motor operating synchronously to fall out of synchronism. It is of course desirable-to provide synchronous motors with pull-in and pull-out torque characteristics thatare as high as possible, and for many applications" the torque characteristics now available with motors of a given size and expenseare not satisfactory;

SUMMARY OF THE INVENTION Accordingly, it is a general object of 'thepresent invention to provide a reluctance motor which overcomes the disadvantages and shortcomings associated with prior art reluctance motors.

More specifically, it is an object of the invention to provide anexternal-rotor reluctancemotor which exhibits' high pull-in and'pull-out torque characteristics. According to one advantageousconcept of the invention an external-rotor reluctance motor :may comprise. a rotor provided witha plurality of pole portions each having respective pole ends, apole. span'betweensaid.

ends, and a pole middle.

Importantly, according to such concept, the rotor has a given effective yoke cross-section over part of the 'respective pole span and a lesser effective magnetic cross-section in the vicinity of a respective pole middle and oversubstantial portions. of the"associated poler span.

By so designing the effective yoke cross-sectionof the rotor, portions of increased magnetic reluctance for quadrature-axis flux may be significantly increasedyat desiredulocations, with resulting better distributiomof .flux in the air gap. The better distribution of flux can result in improved torque characteristics.

According to another. advantageous conceptof the invention, the rotor is provided over asubstantialportion of its individual pole spans vwitha plurality of'nonmagnetic portions of differing size and/or configuration and spaced from the outer periphery of the rotor These non-magnetic portions serve to reduce the' given effective yoke cross-section of the rotor to a lesser. effective yoke cross-section, and thereby increase the magnetic resistance of the rotor at certain locations.

With this constructional possibility, flux-path portions of magnetic materialare provided between respectively adjoining non-magnetic portions provided'in the rotor yoke. In addition, the rotor is provided at its inner periphery with radially inwardly projecting teeth, andad# vantageously the flux-path portions between, adjoining.

non-magnetic portions may be more-or-less aligned.

with neighboring rotor teeth. This will be illustrated and explained in detail.

Also, it is contemplated according to the invention. to vary the effective magnetic cross-section of the rotor yoke in such manner that the cross-section is smallest near the respective pole middles and increases'substantially continuously toward the respective-pole ends., This also will'be shown and explained in detail.

Finally and very importantly, is is noted that withthe i invention it is possible to so dimension the magnetic flux-path portions located on either side of therespective pole-middles that, during idling of the synchronous motor, these flux-path portions and/or the neighboring yokeportions will be substantiallyin the saturation-region, though not necessarily far into the region of oversaturation. In this way it is possible to provide sufficient magnetic cross-section for the direct-axis flux components, while at the same time providing a high'magnetic resistance for the undesired quadrature-axis flux components, especially in the region of the pole'middles. With such design, flux irregularities can be consdierable reduced.

The novel features which are considered as characteristic for the invention are set forth inparticular in the appended claims. The invention itself, however; both as to its construction and its method of operation,

together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read'in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal section through'a reluctance I developed rotor of FIG. 9;

I motor embodying theprsent invention, and taken on" line I"I of FIG. 2;

' FIG. 2 is a transverse s ection through the reluctance motor of FIG. 1, taken on line II-II of FIG. 1;

FIG. 3 illustrates a first modification of the rotor configuration shown in FIG. 2;

FIG. 4 illustrates a second modification of the rotor I the rotor FIG. 9 illustrates in developed view a rotor according to the prior art;

FIG. 9a is a graph of total flux,

FIG. 10 illustrates in developed view a rotor accordv,ing to the present invention;

plotted alongside the v The stator 11 is provided in'conventionalmanner,

with slots 32 (see FIG. 2) in which is provided a conventional three-phase winding, whose left winding end.

is shown in FIG. '1 at 33 and whose right winding end is shown at 34. A three-wire supply cable 35 is connected with the right winding end 34 and is fixedly secured to flange 14 by means of a clamping arrangement 36, or the like. The flange 14 and stator 11 are fixedly connected and cannot turn with respect to each other.

in transverse section. In the event that the rotor is formed of a plurality of stacked substantially annular rotor plates, FIG. 2 would illustrate the general configuration 'of one such rotor plate. 4

It will be seen that the rotor 12 of FIG. 2 is provided with a plurality of conductor slots 40-47 and 41 47' of different depth and width. The rotor illustrated in FIG. 2 is that of a two-pole reluctance motor, each pole of which has a pole span of .1 80 mechanical degrees. The pole middle ,of the pole illustrated in FIG. 2 is designatedM, and the respective pole ends, at the extremes of the pole span, are accordingly spaced from the pole middle M by 90 mechanicaldegrees'each.

The deepest slot 40 is positioned in the vicinity of the pole middle, and the depth of successive slots to either.

' side of the pole middle decreases continuously in direc- FIG. lla illustrates a rotor configuration according to the prior art;

7 FIG. 11b illustrates a rotor configuration according to' the invention; and

FIG. llc is a graph of torque characteristics experimentally determined for the rotor configurations of FIGS. 11a and 11b.

DESCRIPTION OF THE PREFEI IRED EMBODIMENTS FIG. 1 illustrates in sectional view an external-rotor reluctance motor 10, which'has a fixedly mounted internal stator' 11 with stator laminationsll', as well as a rotor 12 with rotor laminations 12', the latter being mounted for rotation. about the stator. The stator 11 is fixedly secured on a support pipe 13 by pressure-fitting, or in other suitable manner, and the support pipe 13 in turn is cast with, or otherwise connected with and supported by a flange 14. The flange 14 is shown provided with a bore 15 which may be used for mounting the motor onto an apparatus to be driven. The various details of construction shown in FIG. 1 are provided merely for the sake of concrete illustration, and do not form part of the invention.

Towards the right in FIG. 1, the flange 14 is provided with a bore 16 in which is mounted the outer run of a ball bearing 17, whose inner run mounts the right end of a shaft 18. A circlip 20 and ring 21 serve to retain the right end face of the ball bearing 17.

The shaft l8extends through the entire length of the support pipe 13 At its left end it is mounted in a second ball bearing 24 and supports at this end a sleeve mounting 25 in force-fit relationship therewith. The mounting v sleeve 25, which may be made of steel, in turn supports tions 12'.

tion towards the respective ends. At the same time, the width of slots increases in direction towards the pole ends, so that the-teeth 64', 6'5, 64", 65", etc., are accordingly narrowest in the region of the pole ends and widest in the region of the pole middles. The width of tooth 64' for example, is designated B and will be seen to considerably exceed the width b of a tooth located near the left-hand pole'end. The various .slots are each filled with non-magnetic material of good electrical conductivity, such as for instance aluminum or aluminum alloy, whereas the remainder of the rotor 12, in-

in thickness and width of the slots just described, it willbe seen that the rotor has an effective magnetic crosssection which varies in circumferential direction of the rotor, and which is smallest in the vicinity of a pole middle and greatest in the vicinity of the respective pole ends. In FIG. 2 the rotor yoke portion of smallest effective cross-section is designated50.

Because, according to the invention, the effective cross-section of the rotor yoke is deliberately narrowed not only in the immediate vicinity of the pole middle but over a substantial percentage of the pole span, the pull-in and pull-out torque characteristics of the reluctance motor in which the rotor of FIG. 2 is incorporated, are very significantly improved. A rigorous ex- 1 planation of this unexpected improvement is not yet available, but it is presently thought that the improvement may possibly be due to the cooperation of two factors:

l. The deliberate reduction of the effective rotor yoke cross-section over a substantial percentage of a pole span results in a considerable increase of the FIG. 2 illustrates a rotor according to the invention magnetic resistance presented to quadrature axis flux, and the fluxtdistribution in .the air. gap'rbetween rotorand statoris therebysignificantly improved, approximately more' nearly the sinusoidal .fluxidistribution desired. 2. Flux irregularities (induction .humpsand'dips) in the inductive characteristics of theair gap. are comfpensated. This effect will be explained somewhat more clearly with reference to-FIGS. 7-1-0. The slots 4147, etc., of the rotorshown in' FIG. 2

are, asalready mentioned, filled with aluminum'oranother suitable material, and the conductors thusly formedin theslots are joined at their left:ends.(in. FIG. 1) by short-circuit ring.29 and at their'rightends (in FIG. 1) by short-circuit ring 51, the connection effected advantageously beingboth electrical andmechanical. In conventional manner, the 1 short circuit rings 29=and 5 l, the aluminumconductors-in the rotor slots,.and eventhe arms1'27,.28.;etc.,.andthe mounting ring26 may be. cast in a. single operation, sotasto'produce a unitary rotor 1e .having great stability and solicity, and. constitutingahousing for "the motor 10.

In thosezsituation where the rotoris.to'be'constructed froma plurality of stacked. rotorplates, itmay bedesirable. for. reasons of. economy to employ. prefabricated rotor plates of the type'commercially available but hole. 58-are of lesser sizeand reduce the yoke crosssection'not as much as the hole'58. I

.It should be understood that anindividual'rotor'will be formed of a-pluralityof rotor plates such as that shown in FIG. 3, and that such rotor. plates will be stacked with their respective slots and yoke reducing holes in registry so as to define elongated cavities extending longitudinally of the rotor. These. slots and holes, and the cavities which they define-,tmay in conventional manner be filled with aluminum, or-another non-magnetic material .havingxgood. electrical conduc- 1 tivity. After such filling, the non-magnetic material fillwhich, however, are not constructedaaccording to the concepts of thepresentinvention, Ineparticular, .it may bedesired to employ rotorplatesoffthe. typeusedin the rotorsof a synchronousmachine.

Specifically, itmay. be'desired' to employ annular rotor plates each of which. is, provided along its inner periphery with conductor-accommodating slots of identical configuration. Suchrotorplates. have aneffective yoke cross-section. which issubstantially constant in circumferential direction.

Rotor plates of this type may be employedin the present invention, if suitably modified, therebyavoiding the necessity of producing specially-designated, and

.thereforeexpensive rotor plates. FIGS. 3 -55 illustrate several suitable modifications.

Theconventional rotor plates of-FlGS.-3 -6=.each'have along their respective inner peripheries a plurality of slots intended to'accommodate. conductors, and which are-defined between respective adjoiningteeth59, 62, etc., which latter project in radially inwarddirection. FIG. 6. depicts an individual slot '55 ofgenerallyquadratic configuration and havinga relatively'narrow slot gap 56, which may for example be of awidth of:=approximately 0.8 mm.

FIG. 3 shows a first possible.modificationaccording to theinve'ntion of a commercially availablerotor'plate of conventional. type. Before vmodification, th'eorotor plate has agiven effective yokecross-section which .is

substantially constant in circumferential direction. The modification consist in stampinga plurality of holes58, 60,60, 61, 61, etc. in the rotor plate,.the-holes-having different dimensions and :beingso located/as to provide .thecross-section design of the present invention, de-

spite theprovision of identical conductor slots. "The largest hole is designated58 andis spaced from the rotor outer periphery'by a smalldistance'corresponding:to the narrowest-width of rotortportion 6.6, and it will be appreciated that because this largest hole is provided in the vicinityofthe pole middleM,mthe effective yoke cross-section of the rotor-is most reduced .atthis vicinity. The holes 60, 60' and 61 6 1' to eitherside of ing the slots and yoke reducing holes will constitute non-magnetic rotor portions having the configuration of the spaces filled.

With thisin mind, it is particularly advantageous if: at least some ofthestamped holes 58, 60,"6f1,:etc., intersect neighboring conductor slots, so as to provide,

when a plurality of rotor. platesaare stacked, elongated cavities which are of substantial cross-section and which accordingly are'easier to fill with .aluminum' or another suitable material.

The yoke reducing holes 58, 60, 61,-etc., have been illustrated as being circular, but it may be desired to stamp holes of another configuration.

In the rotor plate of FIG. S'adjacent ones'ofthe yoke reducing' holes are separated by magnetically conductiveflux path'portions designated 64,64', 65,-65l', etc., Itwill'beqappreciated that when ultimately. a" plurality of rotor plates are stacked with their: holes and slots 65 will be superimposed andthus extend, inthe-zcom- 64, '65, etc., separating adjacent ones of 'the. stamped yoke reducing holes will be more-or-less aligned-with respective neighboring rotor teeth 62, 62', 63, 63,'etc.

Likewise it has been found advantageous-to so dimensionand. position the holes 58, 60, etc., of thei'rotor plate'that the flux path portions will have anveffective cross-section at most equal to the effective crosssectional dimension of respective neighboringteeth62, 63,.etc.

By so dimensioning theyoke reducingholesahdflux path portions with respect to the effective crosssectional dimension of neighboring rotorteeth itis possible to just achieve saturation of the flux pathportions when the reluctance motor is operating in idling. condition; accordingly in loaded condition ofthernotor, un-

desired'induction irregularities in the air gap can be" avoided or significantly reduced, aswillb further explained below.

Still discussing the modified rotor plate of FIG. .3,'--.it will be noted that the desired decrease of 'the'rotor tooth width- (explained with reference to FIG...2,previously.) can be achieved, despite thepresenceof preformed, indentically spaced rotor'teeth by punching. out or otherwise producing auxiliary holes'i'68 which' serve to narrow'the effective tooth width in the vicinity of pole ends. The punched holes 68 may be produced by circular punches .whose punch configuration is indicated in FIG. 3 by dotted-lines at holes 68, or the holes 68 may be produced in any other suitable fashion.

FIG. 4 illustrates a further modification of a commercially available rotor plate of conventional type. In particular, it will be appreciated with respect to the yoke reducing hole 58 of the rotor plate in FIG. 3, that the radial spacing between the hole 58 and the rotor outer periphery, as well as the radial spacing between the hole 58 and the adjoining conductor slot, is quite small, and that the rotor is accordingly greatly. weakened at these locations. In the interest of mechanical strength,

it is desirable to make at least the largest hole, i.e., the hole in the vicinity of the pole middleof quadratic configuration, as done with the yoke reducing hole70 in FIG. 4. With such configuration, or the equivalent, sizable rotor portions 73 and 74 are left between the hole and rotor periphery, and between the hole and conductor slot, so as to significantly increase the mechanical strength of the rotor at these locations over that'associated with the corresponding rotor portions in FIG. 3. That the rotor portions 73 and 74 be possessed of a certain mechanical strength is important, especially because after prolonged use of the reluctance motor of the invention, the mechanical structure of the aluminum conductors filling the aligned conductor slots will undergo changes which may alter its physical form. It will be appreciated that the increased mechanical strength thus achieved will decrease the magnetic resistance of the rotor at the pole middle only a little below that of the similar construction in FIG. 3. Bynarrowing the effective yoke crosssection of the rotor over a substantial percentage of the pole span, the design of the invention makes possible theconjunction of both of these requirements, in contrast to known arrangements according to which the desired high qudrature-axis reactance could be achieved only with very thin, mechanically weak rotor portions (having forexample a thickness of 0.4 mm; see FIG. 11) The greater thickness of portions 73, 74 in FIG. 4 results in improved performance during asynchronous operation, too.

FIG. 5 illustrates a further design possibility. According to this modification, yoke reducing holes 76 are punched out, or otherwise produced, in commercially available rotor disks. The yoke reducing holes 76 are of symmetrical'configuration and have in outline the general shape of a flying bird. The holes 76, importantly, are each positioned at a pole middle and, in this embodiment, the hole 76 intersects conductor slot 59, and is widest at the pole middle M so as to effect at the v pole middle the greatest reduction in the effective magnetic cross-section ofthe rotor yoke. The hole 76 is so 'configurated that the rotor yoke cross-section increases continuously in direction towards the respective pole ends, and it will be seen thatthe cut-out or hole 76 extends over a substantial percentage of a pole span.

It is important'to note, with respect to the modification of FIG. 3, that the hole 76 and rotor teeth 62, 62,

63, 63' define magnetic fluxpath portions77, 77, 78, 78', which are so designed as to be magnetically saturated when the motor is in idling condition. The embodiments of FIGS. 3-5, like the embodimen of FIG. 1, affords greatly increased pull-in and pull-out torque, and it must be presumed that such increase,

too, is the result of an increase of magnetic resistance tribution, especially in the motor air-gap. I

FIG. 7-10 will serve to bring out somewhat more clearly the theoretical aspects of the present invention.

It is customary, in analyzing the performance of reluctance motors, to divide the motor flux into directaxis and quadrature-axis components, i.e., into. flux components d1,- and respectively, representing the flux passing inthe direction of least reluctance and that passing in the direction of greatest reluctance.

FIG. 7a illustrates the direct-axis componentof flux which is in phase with the direct-axis component of magnetomotiveforce 0,. The rotor, illustrated in FIG. 7, in developed view, is one constructed according to the prior art, and provided with only a single hole 82 serving to narrow the yoke cross-section in the vicinity of a pole middle M. The prior art rotor illustrated has identical conductor slots 81, and it will be seen from the Figure that the direct-axis component of flux (,6, (represented by flux-lines 83) is'split into two components by the hole 82. y

FIG. 8 shows in developed view the rotor of FIG. 7; FIG. 7a is graph of quadrature-axis flux 41, and quadrature-axis mmf 0,, plotted alongside the developed rotor. This quadrature-axis flux qb -has a dip in the region of the pole middle M. By way of explanation, it is noted that the flux is in effect diverted by hole 82 and flows back at least in part to the non-illustrated stator, this being indicated by flux-lines 84 and 85. Also, some of the flux is diverted by the hole 82 and flows out of the rotor and back through neighboring rotor teeth, this being illustrated by flux-line 86. These flux components constitute quadrature-axis stray flux. It should be clear that this flux is in effect evidence of an induction hump, which in fact may also be demonstrated by experimental measurements of the induction distribution. FIG. 9, 9a resembles FIGS. 7, 7a and 8, 8a, but presents a superposition of the flux and mmf illustrated in those Figures, and was derived by measurement of the air-gap induction distribution. Curve 87 of FIG. 9a clearly shows an undesirable flux'dip in the region of the pole middle M.

With the construction of the rotor of an externalrotor reluctance motor according to the invention, such induction dip is largely avoided, this having been demonstrated experimentally by measuring the air-gap induction distribution. By employing the rotor of the invention, the flux distribution in the air-gap approaches much more nearly the ideal sinusoidal form.

FIG. 10 illustrates in developed view a rotor according to the invention, the flux-lines there depicted indicating the improved flux distribution achieved. The improvement is due to the effective magnetic-crosssection between the'holes and the outer'periphery of the rotor 90, which is here so dimensioned as to be substantially saturated by direct-axis flux when the motor is in idling condition. If, now, under load conditions a quadrature-axis stray flux is applied, represented in FIG. 10 by flux-lines 97, the magnetic resistance of the ferromagnetic rotor material to such quadrature-axis stray flux-will .risef steeply, since the magnetic material will become oversaturated. In this way, theinduction hump 98 of FIG. 9a will be reduced and the dip 99 insreased-ie, the quadrature-axis stray flux will be compensated, the flux distribution improved, and a higher torque afforded.

FIG. 11a illustrates a portion of a rotor accordingito the prior art, FIG. 11b a portion of a rotor according;

toithe'inventionr FlG. 1 1c is a graph showing results of tests of reluctance motorsemploying externalrotorsaccording to the prior art and according torthe-invention, the motors being in other respects identicalandtested under substantially identical running conditions.

In particular, the prior art rotor of F IG. Ila is designated 101' and its rotor yoke is provided'withonly onehole- 102 in the region of pole middle M'which is sepa'- rated from the rotor periphery l03'by a small-radial dis-"" tancexb. The separation b was variedduring the;testafter each determination, by reducing the rotor diame ter by 0.2 mm ata time, on a lathe. The pull-in torque obtained when employing the rotor is represented by curve 104: The curves in FIG. 11c are plotted with the -radial spacing from the rotor periphery as abscissa,

and the measured torque as ordinate. Curve 105 shows the pull-outtorque achieved when using the priorart rotor, fondifferent, radialspacingsz as justexplained. Maximum torque was achieved with the prior art rotor when sponds somewhat to the rotor design of FIG. 3. The

rotor is provided with a series of'five non-magnetic portions 109, 110, 110', 111, 111 servingto reduce the effective crossseiction of ;the rotor yoke. The non-- magnetic portions 109, 110,110, 111, .111. were formed, as described before, by'first punching holes in rotor plates, stacking'the rotor plates with correspondingholes-in registry so as to form elongated cavities,

and then filling such cavities with aluminum or the like to form-non-magneticxyokereducing portions extending in longitudinal direction of the rotor. Likewise; the

conductor slots provided along the inner periphery of the rotor 108 were formed by originally punching slots I (if not already provided) in a plurality of identical rotor plates, these plates then being stacked with corresponding conductor slots aligned so .as to form elon-' gated cavities which,'with the-cavities formed by .the' punched holes;l09 110 etc'., were filled with aluminum, aluminum.alloyor another suitable electriall conductive, non-magnetic material.

With the rotor 108, the radial spacing A between the non-magnetic portion 110 and the rotor periphery 112 was varied during the course'of an experiment. Specifically, after each test measurement, the distance A was reduced by 0.2mm, by filing. In FIG. llc=curve 114 shows thepull-in torque forrotor 108*with various I spacings A, and curve 115 -shows the pull-out torque.

Optimal valves were achievedwhen radial spacing A equalled approximately 1.9 mmJThe pull-in torque wasimproved by approximately 25 percent, and the pullout torque by approximately percent.

It will be appreciated that for a given motor size and construction type, the expedient of the present invention, althoughvery simple, affords greatly improved that the feature of providing yoke reducing =nonmagnetic portions has here been illustrated by providing non-magnetic portions of for instance aluminum; It

will be understood that non-magnetic portions'of any kind-may be provided, and that insome circumstances 1 I the non-magnetic portions may in fact consist of voidsxl. Also, whereas the rotor shell has been-depicted as a sin-: gle cylindrical shell, certain applications'may of course require different configurations. Thus, the exact orien- I tations of poles, pole spans, etc., and the exact orienta-z tions of quadrature-axis and direct-axis flux compor nents shown, is merely for the purposes of illustration; and of course is not tobe consideredlimitingin any. sense. With regard to the saturation of certain fluxpath portions, for example, this important possibility ac cording to the present invention wouldbe no'less signif-f icant in a different rotor configuration.

It ,will-be understood that each of the elements describedabove, or two or more together, may also find 1 a useful applicationin other types of constructions differing from the types described, above.

While the invention hasbeen illustrated and described as embodied in an external-rotor reluctance motor, it is not intended to be limited to the details shown, since various modifications and structural 1 changes may be made without departing in anyway from thespirit of the present invention.

Without further analysis, the foregoing will so fully" reveal the gist of the present invention that others can by applying current knowledge readily adapt it for vari- I ous' applications without omitting features that, from 'the'standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects'of this invention and, therefore, such adaptations'should-and are intendedto be comprehended within the'meaning and'range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

cross-section over part of the respective'span and a lesser effective yoke cross-section in the vicinity of a respective pole middle and over a substantial portion of the associated pole span, and wherein said'rotor'has an outer periphery, and wherein said rotor is provided 1 over a substantial portion of a respective polespan with a plurality of circumferentially spaced yoke-reducing non-magnetic portions of different'dimensionsspaced from said outer periphery and servirigtoreduce said I given effective yoke cross-section to said lower effe"c=.

tive yoke cross-section, and wherein said rotorcomprises circumferentially spaced magnetically conduc tive flux-path portions separating adjacent ones of said non-magnetic portions and wherein said non-magnetic portions are largest in the region of the respectivepole middle and smaller in the other regions of the respective pole span. a

'2. A motoras defined in claim 1, wherein'said nonmagnetic portions are largest in the region of the-r e--' spective pole middle and successively smaller in direc- 1 tion towards the respective pole ends.

3. A motor as definedin claim 1, said rotor being provided with slots filled with electricallyconductive nonmagnetic material forming a squirrel-cage winding and said rotor having an inner periphery,andfurther having -a radially inwardly projecting teeth provided in the re gion-of said inner periphery; and wherein at least some I of said magnetically conductive flux-path portions'are" substantially aligned with and merge into respective neighboring teeth.

4. A motor as defined in claim 2, said rotor being provided with slots filled with electrically conductive nonmagnetic material forming a squirrel-cage winding, and said rotor having an inner periphery, and further having radially inwardly projecting teeth provided in the region of said inner periphery; and wherein at least some of said magnetically conductive flux-path portions are substantially aligned with respective neighboring teeth and merge into the same.

5. A motor as defined in claim 4, wherein at least some of saidnon-magnetic portions merge with neighboring slots of said squirrel-cage winding.

6. A motor as definedin claim 3, each of said teeth having an effective respective cross-sectional dimension and each of said magnetically conductive flux-path portions having a respective effective cross-sectional dimension, and wherein the effective cross-sectional dimension of at least some of said flux-path portions is substantially equal to the effective cross-sectional dimension of respective neighboring teeth.

7. A motor as defined in claim 4, wherein said fluxpath portions of respective pole portions of said rotor to either side of the, respective pole middle are so dimensioned that during idling of said motor the majority of said flux-path portions are in magnetic saturation.

8. A motor as defined in claim 1, wherein said fluxpath portions of respective pole portions of said rotor toeither side of the respective pole middle are so dimensioned that during idling of said motor the majority of said flux-path portions are in magnetic saturation.

9. A motor as defined in claim 8, said inwardly projecting teethbeing provided in the region of said rotor inner periphery on a substantial part of the pole span of at least one pole portion, and each of said teeth having a respective cross-sectional dimension, and wherein 10. A motor as defined in claim 1, wherein said rotor I comprises a plurality of stacked substantially identical rotor plates each having a plurality of yoke-reducing cut-outs, said plates being stacked with said cut-outs in registry, and said cut-outs forming yoke-reducing cavities extending lengthwise of said rotor; and wherein said yoke-reducing non-magnetic portions are accommodated in respective ones of said yoke-reducing cavities, wherein said non-magnetic portions accommodated in said cavities consist at least substantially of electrically conductive material, and wherein said conductive material is aluminum.

1 l. A motor as defined in claim 1, wherein said rotor comprises a plurality of stacked substantially identical rotor plates each having a yoke-reducing cut-out per pole, each yoke-reducing cut-out being located in the vicinity of the respective pole middle and extending over a substantial portion of the respective pole span, said plates being stacked with said yoke-reducing cutdefining an axis of rotation and a magnetic external: rotor core having a generally annular transverse cross- 1 section with a radially inner periphery and a radially outer periphery, said external-rotor core surrounding saidstator and being carried by said mounting means for rotation about saidaxis, said magnetic externalrotor core including a plurality of circumferentially spaced flux path portions extending along the lengthof said external-rotor core and forming therebetween a plurality of circumferentially spaced slots extending along the length of said external-rotor core, with said flux-path'portions furthermore extending in direction from the inner periphery of said external-rotor core towards the outer periphery of said external-rotor core, and a yoke section extending in circumferential direction of said external-rotor core and joining said fluxpath portions together at the radially outer portion of said external-rotor core, with the radially outermost portions of at least some of said slots being disposed at respective different radial distances from the outer periphery of said external-rotor core to form said yoke section with a width measured indirection radially of said core which has a predetermined smallest value at at least two circumferentially spaced locations to define two direct-axis pole middles presenting maximum reluctance to quadrature-axis flux passing through said locations in direction circumferentially of said external-rotor core, and has a predetermined largest value at at least two further circumferentially spaced locations to define at least two quadrature-axis pole ends presenting minimum reluctance to direct-axis flux passing through said further locations in direction circumferentially of said external-rotor core.

13. A motor as defined in claim 12, wherein those of said slots located to either side of said pole-middles are successively disposed at increasing radial distances from the outer periphery of said external-rotor core, so that said yoke section has a width measured in direction radially of said core which increases in direction circumferentially away from said pole-middles.

14. A motor as defined in claim 12, wherein said slots include a plurality of first slots located at the radially inward portion of said external-rotor core and a second plurality of slots located radially outwards of said first slots and discrete from said first slots, and wherein said second slots are located to either side of said polemiddles, and wherein the width of successive ones of said second slots measured in direction radially of said core decreases in direction circumferentially away from the respective pole middles.

15. A motor as defined in claim 12, wherein said slots include a plurality of circumferentially spaced first slots located at the radially inward portion of said externalrotor core, and wherein there is located at each of said pole middles a second slot having a width measured in direction radially of said external-rotor core which decreases in direction circumferentially away from the respective pole middle, and having an angular span measured with respect to said axis including a plurality of said first slots. 5

16. A motor as defined in claim 12, wherein said slots are filled with non-magnetic electrically conductive material forming a squirrel-cage winding.

17. A motor as defined in claim 14, wherein at least some of said second slots are substantially aligned in direction' radially of said external-rotor core with some of said first slots.

'fective width of said flux-path portions measured in direction circumferentially of said external-rotor core is greatest at said pole middles and smallest at said pole ends. 

1. An external-rotor reluctance motor having a rotor provided with a plurality of pole portions each having respective pole ends, a pole span between said ends and a pole middle, said rotor having a given effective yoke cross-section over part of the respective span and a lesser effective yoke cross-section in the vicinity of a respective pole middle and over a substantial portion of the associated pole span, and wherein said rotor has an outer periphery, and wherein said rotor is provided over a substantial portion of a respective pole span with a plurality of circumferentially spaced yoke-reducing non-magnetic portions of different dimensions spaced from said outer periphery and serving to reduce said given effective yoke cross-section to said lower effective yoke cross-section, and wherein said rotor comprises circumferentially spaced magnetically conductive flux-path portions separating adjacent ones of said non-magnetic portions and wherein said non-magnetic portions are largest in the region of the respective pole middle and smaller in the other regions of the respective pole span.
 2. A motor as defined in claim 1, wherein said non-magnetic portions are largest in the region of the respective pole middle and successively smaller in direction towards the respective pole ends.
 3. A motor as defined in claim 1, said rotor being provided with slots filled with electrically conductive non-magnetic material forming a squirrel-cage winding, and said rotor having an inner periphery, and further having radially inwardly projecting teeth provided in the region of said inner periphery; and Wherein at least some of said magnetically conductive flux-path portions are substantially aligned with and merge into respective neighboring teeth.
 4. A motor as defined in claim 2, said rotor being provided with slots filled with electrically conductive non-magnetic material forming a squirrel-cage winding, and said rotor having an inner periphery, and further having radially inwardly projecting teeth provided in the region of said inner periphery; and wherein at least some of said magnetically conductive flux-path portions are substantially aligned with respective neighboring teeth and merge into the same.
 5. A motor as defined in claim 4, wherein at least some of said non-magnetic portions merge with neighboring slots of said squirrel-cage winding.
 6. A motor as defined in claim 3, each of said teeth having an effective respective cross-sectional dimension and each of said magnetically conductive flux-path portions having a respective effective cross-sectional dimension, and wherein the effective cross-sectional dimension of at least some of said flux-path portions is substantially equal to the effective cross-sectional dimension of respective neighboring teeth.
 7. A motor as defined in claim 4, wherein said flux-path portions of respective pole portions of said rotor to either side of the respective pole middle are so dimensioned that during idling of said motor the majority of said flux-path portions are in magnetic saturation.
 8. A motor as defined in claim 1, wherein said flux-path portions of respective pole portions of said rotor to either side of the respective pole middle are so dimensioned that during idling of said motor the majority of said flux-path portions are in magnetic saturation.
 9. A motor as defined in claim 8, said inwardly projecting teethbeing provided in the region of said rotor inner periphery on a substantial part of the pole span of at least one pole portion, and each of said teeth having a respective cross-sectional dimension, and wherein the respective cross-sectional dimension of teeth located in the vicinity of the pole middle is greater than the respective cross-sectional dimension of teeth located in the vicinity of the respective pole ends.
 10. A motor as defined in claim 1, wherein said rotor comprises a plurality of stacked substantially identical rotor plates each having a plurality of yoke-reducing cut-outs, said plates being stacked with said cut-outs in registry, and said cut-outs forming yoke-reducing cavities extending lengthwise of said rotor; and wherein said yoke-reducing non-magnetic portions are accommodated in respective ones of said yoke-reducing cavities, wherein said non-magnetic portions accommodated in said cavities consist at least substantially of electrically conductive material, and wherein said conductive material is aluminum.
 11. A motor as defined in claim 1, wherein said rotor comprises a plurality of stacked substantially identical rotor plates each having a yoke-reducing cut-out per pole, each yoke-reducing cut-out being located in the vicinity of the respective pole middle and extending over a substantial portion of the respective pole span, said plates being stacked with said yoke-reducing cut-outs in registry so as to form yoke-reducing cavities extending lengthwise of the rotor; and further including portions of non-magnetic material accommodated in said yoke-reducing cavities.
 12. An external-rotor reluctance motor comprising in combination a stator provided with a stator winding; an external-rotor assembly comprising mounting means defining an axis of rotation and a magnetic external-rotor core having a generally annular transverse cross-section with a radially inner periphery and a radially outer periphery, said external-rotor core surrounding said stator and being carried by said mounting means for rotation about said axis, said magnetic external-rotor core including a plurality of circumferentially spaced flux path portions extending along the length of said external-rotor Core and forming therebetween a plurality of circumferentially spaced slots extending along the length of said external-rotor core, with said flux-path portions furthermore extending in direction from the inner periphery of said external-rotor core towards the outer periphery of said external-rotor core, and a yoke section extending in circumferential direction of said external-rotor core and joining said flux-path portions together at the radially outer portion of said external-rotor core, with the radially outermost portions of at least some of said slots being disposed at respective different radial distances from the outer periphery of said external-rotor core to form said yoke section with a width measured in direction radially of said core which has a predetermined smallest value at at least two circumferentially spaced locations to define two direct-axis pole middles presenting maximum reluctance to quadrature-axis flux passing through said locations in direction circumferentially of said external-rotor core, and has a predetermined largest value at at least two further circumferentially spaced locations to define at least two quadrature-axis pole ends presenting minimum reluctance to direct-axis flux passing through said further locations in direction circumferentially of said external-rotor core.
 13. A motor as defined in claim 12, wherein those of said slots located to either side of said pole-middles are successively disposed at increasing radial distances from the outer periphery of said external-rotor core, so that said yoke section has a width measured in direction radially of said core which increases in direction circumferentially away from said pole-middles.
 14. A motor as defined in claim 12, wherein said slots include a plurality of first slots located at the radially inward portion of said external-rotor core and a second plurality of slots located radially outwards of said first slots and discrete from said first slots, and wherein said second slots are located to either side of said pole-middles, and wherein the width of successive ones of said second slots measured in direction radially of said core decreases in direction circumferentially away from the respective pole middles.
 15. A motor as defined in claim 12, wherein said slots include a plurality of circumferentially spaced first slots located at the radially inward portion of said external-rotor core, and wherein there is located at each of said pole middles a second slot having a width measured in direction radially of said external-rotor core which decreases in direction circumferentially away from the respective pole middle, and having an angular span measured with respect to said axis including a plurality of said first slots.
 16. A motor as defined in claim 12, wherein said slots are filled with non-magnetic electrically conductive material forming a squirrel-cage winding.
 17. A motor as defined in claim 14, wherein at least some of said second slots are substantially aligned in direction radially of said external-rotor core with some of said first slots.
 18. A motor as defined in claim 12, wherein said flux-path portions and said slots are so dimensioned that in the region of said pole middles at least some of said flux-path portions are substantially magnetically saturated when said motor is idling.
 19. A motor as defined in claim 12, wherein the effective width of said flux-path portions measured in direction circumferentially of said external-rotor core is greatest at said pole middles and smallest at said pole ends. 